This is a prospective case-control study. At each of the six study centers, cases of early-onset GBS disease(defined as isolation of GBS from a normally sterile body fluid in a newborn whose illness began within seven days after birth) are identified by active daily surveillance of microbiology laboratories and neonatal intensive care nurseries. Controls are infants who are surface colonized with GBS at birth but do not develop GBS disease. The target for enrollment therefore is 140 cases, 560 heavily colonized controls (>2 sites) and 840 lightly colonized controls (<2 sites). Sera from cases and controls will be assayed using ELISA method for serotype- specific IgG and IgG subclass antibodies to each serotype. Epidemiological data are collected on cases and controls by chart abstraction. In addition, a retrospective case-control study to evaluate the effectiveness of intrapartum antibiotic prophylaxis against early-onset GBS disease in neonates was conducted. The case accrual and control assembly began in July 1995, and ended on June 30, 1999. A total of 132 cases were accrued (52 were serotype Ia, 27 type III, and 23 type V). The number of case accrued for serotype Ia and III should provide reliable estimate of protective levels of antibody. A total of 963 heavily colonized infants and 592 lightly colonized infants were assembled as controls. Maternal sera were available in over 95% of cases and controls. Serological assay for type Ia antibody is underway. - Group B Streptococcus - Human Subjects